1. Field of the Invention
The present invention relates to laser transmitters and more particularly concerns a laser transmitter that provides improved modulation of the transmitted beam. The invention is useful in laser transmitters used for communication or in those used as radars.
2. Description of Related Art
Frequency modulation of a laser is employed in several types of radars, such as FM chirp lasers, for example, and is also employed for communication. Various arrangements have been employed for frequency modulation, including the use of a piezoelectric transducer mounting one of the cavity reflectors and energized with a frequency modulating signal. The piezoelectric transducer, however, is expensive and consumes a large amount of power. Further, most piezoelectric tranducers have resonant frequencies, in the 1-20 kHz range, that limit useful maximum frequency and may cause instabilities in optical alignment.
Another form of frequency modulation, one employed for a frequency chirp laser radar, is described in U.S. Pat. No. 4,666,295 to Duvall et al for Linear FM Chirp Laser and in U.S. Pat. No. 4,660,206 to Halmos et al for Chirp Laser Stabilization System, both assigned to the assignee of the present application. In the system described in the Duvall et al patent, an electro-optic modulator crystal is mounted inside the laser cavity and subjected to a high voltage modulating signal which produces rapid switching of laser output frequency. The intra-cavity electro-optic modulator employed for frequency control in this system introduces large cavity losses, and thus reduces laser power by over fifty percent. Moreover, frequency control of the arrangements of both patents is very costly, accounting for as much as one-third of the cost of a laser radar transmitter.
U.S. Pat. No. 4,464,758 refers to a method for implementing phase or AM laser radar in Q-switched laser radar. The standard CW (continuous wave) AM laser radar suffers from range ambiguity. It ranges off the incremental phase difference between the reflected and transmitted beams. It measures all phase difference as being between 0.degree. and 360.degree. C. and cannot distinguish between a target within one AM wavelength from the sensor and a second target, an integral number of AM wavelengths farther away. By comparison, a Q-switched radar can measure the absolute range, but the long (200 ns) pulse limits the range accuracy. U.S. Pat. No. 4,464,758 combines the two techniques, using AM modulation to produce a sharp edge in the center of the Q-switched pulse. The sharp edge can be tracked to the same sort of accuracy as the AM System.
Applicant's system does not require the added complexity of a Q-switched operation and, unlike the device of U.S. Pat. No. 4,464,758, will operate CW. Practical Q-switched operation requires either the same electro-optic modulator used for standard pulsed laser radar (see for example U.S. Pat. No. 4,498,179) or a complicated pressure regulating circuit that the U.S. Pat. No. 4,464,758 does not mention in its passive Q-switched approach.
Acousto-optic devices have been used for frequency modulation, but this scheme too is costly and inefficient. Depending on the acousto-optic drive, efficiency may be less than ten percent at CO.sub.2 wavelengths.
For a laser radar system, a pair of lasers in the transmitter is employed, utilizing one laser as a local oscillator with precise frequency control and a second transmitter laser with some means for effecting amplitude or frequency modulation. Where an electro-optic modulation crystal is employed, the transmitter is very costly, because the laser must operate at a carefully controlled carrier frequency to allow heterodyne detection of the received reflected signal. In the acousto-optic format to provide Doppler velocity in radar application, the carefully controlled carrier frequency is also necessary. This precise control of frequency requires an expensive frequency control unit, generally consisting of a piezoelectric crystal, an optical power sensor, and an electronic servo with a high voltage output. The electro-optic and acousto-optic devices an associated electronics are also expensive. The low efficiency modulation of these arrangements often results in unacceptably high power consumption.
Where a laser is used as a transmitter of a communication system, electro-optic frequency modulation of a carbon dioxide CO.sub.2 laser may be employed in the transmitter laser to generate the signal. In the receiver a corresponding laser is required as a local oscillator to decode the signal, because frequencies of the transmitted information bearing laser beam are too high for the receiver processing electronics. Thus for FM laser communication systems every receiver must have its own local oscillator laser, resulting in high receiver costs and high power consumption. With a battery operated receiver, required presence of a laser local oscillator exacts a high price in power requirements.
Accordingly, it is an object of the present invention to provide a laser transmitter which avoids or minimizes above-mentioned problems.